Insulin-Arg.sup.B 31-OH, insulin-Arg.sup.B 31-Arg.sup.B 32-OH and other insulins modified C-terminally by bases in the B chain exhibit depot character in drugs without specific additives, such as Surfen.sup.R or protamine. Such insulins, processes for their preparation, agents containing them and their uses have already been proposed (German Patent Applications P 33 26 472.4, P 33 26 473.2, P 33 27 709.5 and P 33 27 928.4). The advantages of such delayed-action insulin formulations are better tolerance and, in particular, in the case of the long-term treatment associated with insulin treatment, the absence of immunogenicity of the depot auxiliary.
In the biosynthesis of insulin, the double-chain insulin is formed from the immediate precursor, the single-chain proinsulin, by enzymatic detachment of the so-called connecting peptide. However, very small amounts of so-called intermediate insulins are also to be found in the crude insulin, isolated as a result of incomplete processing. The structures of these intermediates of insulin biosynthesis have been clarified. They consist predominantly of B31-Arg- and B31-32-Arg-Arg-insulin derivatives.
If proinsulins from cattle, pigs or humans are treated in vitro with trypsin (Chance, Excerpta Medica International Congress Series No. 231, page 292 et seq.), a mixture of B31-Arg-and B32-Arg-Arg-insulins in particular is formed, besides the corresponding desoctapeptide-B23-30-insulins. Reliable separation and high purification of these products is very expensive and requires several ion exchange chromatography steps, which results in product loss. Isolation of preparative amounts from crude insulin for use in therapy is thus not practicable. In addition, the products corresponding to human insulin are not available.
A possible way of isolating such derivatives consists in enzymatically splitting proinsulin, or analogs thereof, prepared by genetic engineering. As mentioned, losses arise from the purification process.
The object of the present invention is to describe a process in which those amounts of insulin derivatives modified by bases which are required for therapeutic use of the novel galenical formulations can be prepared in a simple manner.
The process according to the invention described below achieves the object by enzyme-catalyzed conversion of inexpensive biosynthetic insulin, such as human insulin, or porcine insulin, using suitable peptides to give the desired products.